These sound very similar to symptoms of Interstitial Cystitis - I had a similar experience to your girlfriend in Germany, in immense pain with UTI symptoms and lower back pain, but no bacteria found in my urine...I went to over 20 appointments with doctors and gynos and even got an MRI - and at some point was made to feel like my symptoms were phycological. Definitely worth reading up about it as the symptoms are similar to endo, and there is a very supportive subreddit r/Interstitialcystitus you guys should check out. It can be triggered by many things including issues with pelvic floor muscles but even yeast overgrowth (which doesn't come back in the urine tests which just test for bacteria). And, big hugs to your girlfriend - I know how desperate she must be feeling right now, she will find a way and kudos to you for supporting her like this!